Because some or most (?) folks would prefer the Sub over the 300 assuming everything else equal. That’s the premise.
I'm pretty sure people also said that it was the original Seamaster 300m from the mid 90's which was the watch that forced Rolex to step up their bracelet quality and make significant improvements to match what Omega was doing for less than half the price, whilst still offering a sapphire crystal and 300m water resistance alongside the workhorse 1120 ETA movement that was practically equal in real world use to a Rolex 3000 movement of the time. All those 90's Submariner's generally had over a Seamaster of the same time period was a better bezel, applied white gold indices and the brand name.
Much of the same can be said about the current Seamaster and Submariner (at least 114060) where besides the better bracelet on the Submariner with its excellent tool free adjustment, there's very little between them besides massive price difference.
If you set aside price, most would want the Sub. Like if someone offered you a new Sub or a new Seamaster 300 for free, and the only stipulation is that you can never sell it or gift it, but you can wear it for the rest of your life then it gets donated to charity. Of course you take the Sub.
Ok, so you see my point. In your hypothetical, I would select the Omega. Why? Because I simply don't care for Rolex. I don't find the watches attractive and I don't like the way that the company does business. The resale value of the brand, which admitttedly is second to none, isn't a prime consideration for me whether it's Rolex, Omega, or any other brand. I buy what I like, not what is going to hold its value the best. Like many members here on OF, I own a number of watches including some brands that are objectively better than Rolex. Although I can afford to purchase a Rolex, I don't own one because they simply don't appeal to me. Market value is a consideration, of course, but far from the most important one for many of us. So I don't buy into your argument that 99% of us would choose the Rolex in your hypothetical.